1. Field of the invention
This invention relates to zero-crossing threshold detectors in general and, more specifically, to a digital zero-crossing detector for providing a digital output signal representing the detection location within an asynchronous sampling window.
2. Description of the Related Art
In the data storage arts, binary data is encoded and stored on optical or magnetic media as a series of optical or magnetic transitions. Retrieval of stored data requires a detecting and decoding system in the recording channel to reconstruct the original binary data and synchronous clock from the self-clocking analog signal waveform created by the magnetic or optical transducer adjacent to the storage medium. Practitioners in the art have proposed many solutions to the problems associated with extracting synchronous clock and data from a self-clocking data signal. These problems include discrimination between actual transitions and mere noise pulses and precise reconstruction of the synchronous data clock signal to permit accurate decoding of the phase-encoded data. An important element in such recording channels is the waveform transition or "zero-crossing" detector, which is necessary to accurately determine the precise relative timing or phase of each self-clocking waveform transition.
Analog pulse detectors known in the art suffer from the usual disadvantages of analog electronic apparatus. They are expensive, bulky and subject to calibration drift over time. Moreover, analog pulse detectors are generally suited to a narrow predetermined range of channel data rates, imposing severe channel data rate restrictions on storage media data retrieval systems. Digital implementations of data pulse or transition phase detectors known in the art usually rely on discrete-signal embodiments of the well-known analog detection techniques. For instance, the analog signal waveform is first sampled and digitized using well-known Phase-Locked Loop (PLL) techniques. These samples are then processed digitally to remove unwanted frequency components and to reconstruct the synchronous clock and data. There is a clearly felt need in the art for a fully digital implementation of a channel waveform transition phase detector that can accurately detect self-clocking data pulses in a recording channel data signal waveform over a wide range of data rates. The crucial need is for accurate synchronous data detection at moderate asynchronous sampling rates because high-speed sampling techniques are disadvantageously expensive.
Digital computer programs are known in the art for simulating the functions of analog recording channels but these techniques require high sampling rates, making implementation in real-time hardware expensive and difficult. Some of the difficulty involved in digital implementation can be overcome by reducing the asynchronous analog signal sampling rate. Unfortunately, a reduced asynchronous sampling rate results in increased uncertainty of the zero-crossing detection time. This leads to jitter distortion and increased Bit Error Rate (BER) in the recording channel.
Practitioners in the art have made efforts to reduce jitter at lower sampling rates by improving inter-sample interpolation. For instance, in U.S. Pat. No. 4,412,339, Peter H. Alfke et al disclose a zero-crossing interpolator intended to reduce isochronous distortion in a digital FSK modem. Alfke et al teach improving the precision of zero-crossing detection by adding a high-speed internal clock to step the detector along a linear slope between each input sample pair until a change in sign is detected. Thus, their technique requires the same high speed digital devices that make higher sampling rate disadvantageous. Moreover, although Alfke et al teach the use of digital devices, the output of their zero-crossing interpolator is a simple analog timing gate that is subject to the same sources of analog errors affecting analog zero-crossing detector embodiments.
In U.S. Pat. No. 4,165,491, Arthur P. Geffon discloses a circuit for detecting zero-crossing points in a data signal in the presence of noise. Geffon teaches a pulse-qualification technique for eliminating zero-crossings that presumably arise from noise. He neither considers nor suggests methods for detecting zero-crossings in a digitally sampled signal.
In U.S. Pat. No. 4,749,879, Donald S. Peterson et al disclose a signal transition detection method for finding signal waveform transitions in a binary-encoded analog signal waveform. Peterson et al teach the use of a second differentiation step to provide a second derivative signal that improves the noise immunity of their analog circuit. They neither consider nor suggest means for detecting threshold transitions in a digitally-sampled signal waveform. Other similar disclosures of improved analog detectors may be found in U.S. Pat. Nos. 3,593,166; 3,916,328; 3,955,102; 4,132,909; 4,151,427; 4,268,764; 4,480,200; 4,795,915; and 5,001,364. There is still a clearly felt need in the art for a digital zero-crossing detector that incorporates digital interpolation techniques to provide accurate transition timing outputs at relatively low asynchronous sampling rates. This requirement is especially important in modern digital recording channels that must be entirely implemented on low-power single-chip digital integrated circuits without analog components. The related unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.